We observed mixing between two-electron singlet and triplet states in a double quantum dot, caused by interactions with nuclear spins in the host semiconductor. This mixing was suppressed when we applied a small magnetic field or increased the interdot tunnel coupling and thereby the singlet-triplet splitting. Electron transport involving transitions between triplets and singlets in turn polarized the nuclei, resulting in marked bistabilities. We extract from the fluctuating nuclear field a limitation on the time-averaged spin coherence time T2* of 25 nanoseconds. Control of the electron-nuclear interaction will therefore be crucial for the coherent manipulation of individual electron spins.

We investigate the temperature and voltage dependence of the quantum interference in an electronic MachZehnder interferometer using edge channels in the integer quantum-Hall regime. The amplitude of the interference fringes is significantly smaller than expected from theory; nevertheless the functional dependence of the visibility on temperature and bias voltage agrees very well with theoretical predictions. Superimposed on the Aharonov-Bohm ͑AB͒ oscillations, a conductance oscillation with a six times smaller period is observed. The latter depends only on gate voltage and not on the AB phase, and may be related to single electron charging.Electron interferences in mesoscopic conductors manifest themselves in conductance oscillations which are h / e periodic in the magnetic field B. 1 In conventional metals the visibility I of these Aharonov-Bohm ͑AB͒ oscillations typically amounts to I = G 0 / G Ӎ 10 −3 , where G is the conductance of the sample and G 0 =2e 2 / h the conductance quantum. The visibility can be enhanced to I Ӎ 0.1 by reducing the number of conductance channels, e.g., in nanostructures based on two-dimensional electron systems 2,3 containing only a small number of conductance channels. Recently an electronic analog 4 of the well-known optical Mach-Zehnder interferometer ͑MZI͒ was realized, which employs the onedimensional edge channels in the integer quantum Hall regime. 5 In these devices single channel interference can be realized, while backscattering processes are suppressed. This results in measured visibilities up to I Ӎ 0.6. Such devices open a path for the realization of fundamental two-particle interference experiments in the spirit of Hanbury-Brown and Twiss, 6 as recently proposed. 7 We have realized MZIs similar to those in Ref. 5. A quantum point contact ͑QPC͒ is used to partition an edge channel leaving contact S in two paths. After propagation of the edge channel along the inner and outer edge of a ring-shaped mesa, the two paths are brought to interference at a second QPC, resulting in two output channels D1 and D2 of the interferometer ͑see Fig. 1͒. The phase of the two partial waves can be changed both by magnetic field and by an electrostatic gate G. The measured visibility is much smaller than expected from theory 8 and reported in Ref. 5. Despite this quantitative disagreement, the functional dependence of I on T and V bias fits very well the simple model of Ref. 8. In addition to the AB oscillations we found another type of conductance oscillations, which have a significantly smaller period in gate voltage when compared to the AB period.The mesa was prepared through wet etching of a modulation doped GaAs/ Ga x Al 1−x As heterostructure containing a two-dimensional electron system ͑2DES͒ 90 nm below the surface. At 4 K, the unpatterned 2DES density and mobility were n = 2.0ϫ 10 15 m −2 and = 206 m 2 / ͑V s͒, respectively. Using standard electron beam lithography techniques, we prepared split gates connected by air bridges, defining QPCs of 500 nm length and 220 nm gap width. Th...

We study a double quantum dot (DQD) coupled to a strongly biased quantum point contact (QPC), each embedded in independent electric circuits. For weak interdot tunneling we observe a finite current flowing through the Coulomb blockaded DQD in response to a strong bias on the QPC. The direction of the current through the DQD is determined by the relative detuning of the energy levels of the two quantum dots. The results are interpreted in terms of a quantum ratchet phenomenon in a DQD energized by a nearby QPC.

We observe the total filling factor nuT=1 quantum Hall state in a bilayer two-dimensional electron system with virtually no tunneling. We find thermally activated transport in the balanced system with a monotonic increase of the activation energy with decreasing d/lB below 1.65. In the imbalanced system we find activated transport in each of the layers separately, yet the activation energies show a striking asymmetry around the balance point, implying a different excitation spectrum for the separate layers forming the condensed state.